Many document generating systems convert document data into control signals that operate an ink ejecting print head in a printer, for example, to produce an image of a document with ink drops emitted from the print head. In some of these systems, an electronic version of a document from a personal computer (PC) or other type of computing system is used to produce the document on media, such as paper or film. In other systems, an electronic document is generated by scanning an original hard copy document with a light source to generate reflected light representative of the document. The light signals are converted into electrical signals that may be stored in an electronic memory. The document generating system typically includes an image processor that manipulates the electronic data representing a document to a processed form of the document that is used to produce the hard copy version of the document.
A print engine may be used to manage the subsystems that cooperate to generate a document on media. These subsystems include the image processor and the components that apply or transfer marking material, such as ink, to media to form a document. For example, a direct marking system may include a marking material source, a print head, an image substrate, and a fuser. The marking material source may be an ink cartridge or a solid ink subsystem. Solid ink subsystems have a loader in which sticks of solid ink are loaded and transported to an ink melter that heats the ink sticks to a melting point to generate liquid ink. The liquid ink is collected in a reservoir to supply the print head. Ink cartridge systems are supplied with ink stored in replaceable ink cartridges. When the ink supply within a cartridge is exhausted, the cartridge is removed and replaced with another cartridge.
The print head in a document generating system is typically comprised of a plurality of ink jet nozzles arranged in a matrix. The ink jet nozzles are coupled by capillaries to the ink supply. They also include piezoelectric elements that are selectively excited by electrical signals from the print engine to eject ink from the capillaries onto an image substrate. In some systems, the print head may be a single print head supported on a carriage so the print head traverses back and forth in a horizontal path across the face of the image substrate. In other systems, multiple print heads that remain stationary and cover a portion of the image substrate may be used. For example, four print heads, each one covering half the width of the image substrate, may be arranged in a two by two matrix opposite the image substrate. Some systems may have one or more print heads that cover the entire width of the image substrate. The print engine communicates with the image processor to convert the electronic data of the document into the electrical signals delivered to the ink nozzles of the print head or heads to reproduce the electronic document on the image substrate.
Prior to generation of the electrical signals, the image processor typically filters or otherwise processes the electronic data for a document so the data are in a better form to generate the electrical signals for the driving the print heads. For example, one or more tone reproduction curves (TRCs) may be used to map the input document data from an image gray scale to the gray scale that corresponds to the half tones produced by the print heads. These TRCs are generated by causing the print heads to print a plurality of test half tone patterns. These printed test patterns may be scanned by a sensor array in the document generating system or by an external scanner to evaluate the amount of ink ejected by the print heads. These data provide an indication of the amount of ink ejected by a print head in response to input data corresponding to a uniform gray scale. If the print head produces a darker gray scale than the input data, the input gray scale value is correlated to another gray scale value that results in an output pixel having a grayscale value that corresponds to the original pixel data. The input gray scale value and its correlated value form a datum point on a graph of input versus output grayscale values. A collection of these datum points over a grayscale range forms a TRC. A look up table may be constructed that uses input pixel data as addresses with the correlated gray scale value stored at the address. Using the look up table, input pixel data are mapped to the correlated gray scale values that are used to generate the electrical signals for the print head that produce a document image that corresponds more closely to the input document data.
Another type of input image data processing is error diffusion. In this type of image processing, an image data conversion process includes an error correction factor. For example, a scanned document may produce color data in the red, blue, and green (RGB) system. These data may be converted into a luminance system for generation of image data in the CMYK color space. The image generated on the media using the CMYK data may be evaluated to identify an error between the input data and the resulting color image. The error in a region may be diffused across the region by adjusting the luminance conversion with an error correction factor. Such error diffusion processing methods are known.
The various input image processing methods are intended to map the input-data to data that generate electrical signals for the print head or heads so they eject ink in a manner that better reproduces the electronic version of the document. The processing applied to the image data may even differ between text data and image data or halftone and color data. While these methods improve the correspondence of the output document to the input document, the document generating system components have differences that may require different adjustments. For example, different TRCs may be generated for different print heads because one print head ejects more ink than necessary for a particular gray scale while another print head may print less than needed for the same gray scale. Even within a print head, the piezoelectric elements may vary in their response to the same electrical signal. The different responses of the ink jet nozzles may cause gradients in the output document that are detectable by the human eye.
The variances in the responses of the ink jets impose a significant computational burden on the document data processing. If a TRC, for example, was developed for each ink jet nozzle, application of a different TRC to the pixel data generated for each nozzle requires a substantial number of look up table operations as TRCs are frequently implemented with look up tables. Even if this type of computational overhead could be handled, it would be exacerbated by the variations in the response of a single ink jet. The piezoelectric element in an ink jet nozzle ages over the life of the ink jet printing device. This aging may cause the piezoelectric element to eject less or more ink than it did at the start of its life. Periodic calibration may be able to adjust a TRC or error correction factor to counter this effect of component aging.
Another source of variance that is more problematic is the change in the drop mass ejected by an ink jet as the firing pattern of the jet changes. For example, an ink jet ejector may eject an ink mass if it has fired on the previous ejection cycle that is different from the ink mass if it has not fired on the previous ejection cycle. The drop mass may further depend on whether a drop was ejected on the cycles occurring before the previous ejection cycle. These variances in ink jet nozzle responses make document data processing more difficult, yet compensation for these differences is desirable.